One thing that I remember that really stuck out to me when I read the Anabasis, by Xenophon was when they were traveling through the Kurdish mountains of what modern northern Iraq/turkey.

In the book, a unit of Greek mercenary hoplites gets stuck deep in Persian territory, after their Persian rebel commander is killed. They then have to fight their way back to Greek territory. As a unit of heavy infantry they don't have too much trouble surviving and moving north until they encounter the Kurds.

Xenophon describes them as a barbaric people "ADDICTED TO WAR" and they end up fighting pretty much a running battle the entire time they are moving through the Kurdish area.

Hopefully a real historian will chime in, with more to say.